U.S. Pat. No. 3,922,506 discloses an acoustical testing system for testing acoustical devices by generation of test tone signals at selected frequencies and amplitudes which are transmitted to an acoustical device under test. The output signals, from the device under test, are automatically processed to actuate readout devices that simultaneously indicate, in digital format, sound pressure level in dB's and percent of harmonic distortion of the signals from the device under test. The selection of the frequencies and the amplitude is done manually.
U.S. Pat. No. 4,065,647 is an improvement over the invention in U.S. Pat. No. 3,922,506 by the use of a microprocessor to automatically generate test tone signals at selected frequencies and amplitudes to drive an audiological device under test and then automatically process the output signals from the device to operate readout means to indicate digital information of the sound pressure level in dB's and percent of harmonic distortion of the output signals from the device under test.
Both the systems in the U.S. Pat. Nos. 3,922,506 and 4,065,647 use standard discrete filter networks and RMS detectors to process the signals to provide the sound pressure level and percent of harmonic distortion as digital readout information.